Various hand-held devices are known which utilize a granular purifying agent, such as activated charcoal and/or an ion exchange material for the purification of water. Examples of such devices are: U.S. Pat. Nos. 4,306,971 (Hankammer); 4,895,648 (Hankammer); 4,969,996 (Hankammer); and, 5,049,272 (Nieweg).
Generally, these devices comprise a large funnel-shaped member to hold and channel the water to be purified, a removable filtration cartridge placed in the flow channel of the funnel-shaped member, and a collection beaker for the purified water. The filtration cartridge is ordinarily constructed to allow the water to flow therethrough, and contains the aforementioned granular purifying agent and/or ion exchange material. These filtration cartridges are typically constructed from inexpensive plastics materials and are designed to be discarded after the granular purifying agent loses its effectiveness, typically after about a month of normal household usages (i.e., after about 100 liters of water have been filtered).
The rate of flow of water through the filter cartridge is critical for effective water purification. It is necessary to contain the water within the filtration cartridge for a sufficient time to allow for ion exchange and filtration to occur, yet if the flow rate through the cartridge is too slow, then an insufficient volume of water will be purified, in a time frame which is satisfactory to the user of the filtration system. Certain known filtration cartridges, such as the subject of U.S. Pat. No. 4,895,648 (Hankammer) [equivalent to EP-A-0340382] and published UK Patent Application No. GB-A2 197 647 (Clarke et al.), rely upon an array of water flow openings which are sufficiently small to trap the granular purifying material within the cartridge, while allowing water to pass therethrough. There are disadvantages connected with such a configuration. The size of the water flow openings is necessarily very small in order to trap the granular purifying agent within the cartridge; this sizing results in significant time lag for overall water filtration, since only a small volume of water can pass through the filtration cartridge openings at a given time. Additionally, there is the potential for particles of the granular purifying agent to become trapped in the water flow openings, blocking them and further reducing flow rates. Further problems develop in relation to the venting of air out of the collection beaker through the filter cartridge. One prior art solution has been to design the filter cartridge for use with a hollow venting tube which protrudes upwardly above the top level of the funnel-shaped channelling member, so as to remain at all times above the level of water in the funnel-shaped channelling member. This system is expensive to manufacture, and is unwieldy to use.
Other known water filtration cartridges have adapted generally planar felt-like plastic or fabric secondary filters positioned adjacent the water flow openings in the top and bottom ends of the cartridge to retain the granular purifying agent, which arrangement permits the use of enlarged water flow openings on the outer housing. Such larger openings in turn permit rapid water flow rates through the filtration cartridge. These cartridges are relatively effective, but are expensive to manufacture and require significant additional engineering of the device in order to vary water flow rates for different use applications. Moreover, the top and bottom secondary filters must be sized differently in order to fit the different top and bottom end dimensions of the filtration cartridge. More particularly, the top secondary filter is typically conical, to correspond to the general shape of the cartridge cap, which cap has been so configured to achieve adequate air ventilation. Accordingly each of the top and bottom secondary filters must be separately manufactured using molds or dies of differing sizes and shapes. Further, it is necessary to use some form of fastening means to secure the secondary filters, most notably the top secondary filter, to the top cap of the filtration cartridge. Plastic welding is sometimes used for this purpose, as disclosed in U.S. Pat. No. 5,049,272 (Nieweg). This step further complicates assembly and increases manufacturing costs. Alternatively, the use of top and bottom secondary filters constructed of light, fluffy, loosely packed fibrous matter comprising commingled, non-woven fibres of significant length relative to their diameter has been proposed in published PCT Application Ser. No. WO-A-89/00977. However, the design of this cartridge does not have the conventional conical cap design mentioned above, which design has become an industry standard, and no means are proposed in this reference to hold the top secondary filter within the cap to facilitate assembly as aforesaid.
The above mentioned prior art systems using generally planar felt-like plastic or secondary filters additionally suffer from the following disadvantage. As the filtration cartridges are used over time, settling out and packing of fine particles of the granular purifying agent tends to occur, particularly on the secondary filter adjacent the bottom end of the cartridge. This settling out of fine particulate on the secondary filters has the effect of clogging the secondary filters and thereby slowing down the flow rate of the filtration cartridge over time. The effect is more pronounced if the cartridge is permitted to dry out between uses. Such slowing down of the flow rates is generally undesirable and irritating to the end user.